U.S. patent number 5,739,734 [Application Number 08/782,112] was granted by the patent office on 1998-04-14 for evanescent mode band reject filters and related methods.
This patent grant is currently assigned to Victory Industrial Corporation. Invention is credited to Ming Hui Chen, Song Mu Yang.
United States Patent |
5,739,734 |
Chen , et al. |
April 14, 1998 |
**Please see images for:
( Certificate of Correction ) ** |
Evanescent mode band reject filters and related methods
Abstract
Apparatus and related methods for an easily manufactured
evanescent mode band reject filter that provides high performance
with minimal dependence on critical dimensions. According to one
embodiment, the present invention provides a band reject filter
including a waveguide having an input, an output, a first wall
between the input and the output, and a second wall opposite the
first wall. The first wall is part of a substantially solid first
block, and the second wall is part of a substantially solid second
block. The waveguide is capable of transmitting an electromagnetic
radiation signal from the input to the output, where the signal is
at an operating frequency above a waveguide cutoff frequency. The
band reject filter also includes at least one cavity coupled
directly to the first wall of the waveguide, where the cavity is a
substantially cylindrical cavity formed in the first block.
Further, the cavity operates in an evanescent mode such that the
cavity has a cavity cutoff frequency above the stopband frequency
of the band reject filter. The cavity may have a circular,
elliptical, or substantially rectangular cross-section in some
specific embodiments.
Inventors: |
Chen; Ming Hui (Taipei,
CN), Yang; Song Mu (Taipei, CN) |
Assignee: |
Victory Industrial Corporation
(CN)
|
Family
ID: |
25124995 |
Appl.
No.: |
08/782,112 |
Filed: |
January 13, 1997 |
Current U.S.
Class: |
333/210; 29/600;
333/209; 333/249 |
Current CPC
Class: |
H01P
1/219 (20130101); Y10T 29/49016 (20150115) |
Current International
Class: |
H01P
1/20 (20060101); H01P 1/219 (20060101); H01P
001/219 (); H01P 011/00 () |
Field of
Search: |
;333/208,1,212,227,228,231,239,248,249,250,253 ;29/600 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pascal; Robert
Assistant Examiner: Ham; Senngsook
Attorney, Agent or Firm: Townsend and Townsend and Crew
LLP
Claims
What is claimed is:
1. A band reject filter comprising:
a waveguide having an input, an output, a first wall between said
input and said output, and a second wall opposite said first wall,
said first wall being part of a substantially solid first block,
said second wall being part of a substantially solid second block,
said waveguide capable of transmitting an electromagnetic radiation
signal from said input to said output, said signal at an operating
frequency above a waveguide cutoff frequency; and
at least one cavity coupled directly to said first wall of said
waveguide, said cavity being a substantially cylindrical cavity
formed in said first block, said cavity operating in an evanescent
mode such that said cavity has a cavity cutoff frequency above the
stopband frequency of said band reject filter.
2. The band reject filter of claim 1 wherein said electromagnetic
signal is a microwave or millimeter-wave signal, and said cavity
has a cross-section that is circular, elliptical, or substantially
rectangular.
3. The band reject filter of claim 1 wherein said waveguide is a
rectangular cross-sectional waveguide having side walls
perpendicular to said first and second walls, and said
substantially cylindrical cavity has slightly inwardly slanted
cavity walls substantially parallel to said side walls.
4. The band reject filter of claim 1 wherein said cavity has a
circular cross-section with a diameter that determines the stopband
frequency.
5. The band reject filter of claim 4 wherein said waveguide is a
rectangular cross-sectional waveguide having side walls
perpendicular to said first and second walls, and said
substantially cylindrical cavity has cavity walls parallel to said
side walls.
6. The band reject filter of claim 4 wherein said waveguide is a
rectangular cross-sectional waveguide having side walls
perpendicular to said first and second walls, and said
substantially cylindrical cavity has slightly inwardly slanted
cavity walls substantially parallel to said side walls.
7. The band reject filter of claim 4 further comprising:
at least one tuning stub disposed through said second block and
said second wall and opposite said cavity for impedance matching to
said cavity.
8. The band reject filter of claim 4 wherein said diameter is about
13.5 mm and said stopband frequency is about 14 GHz.
9. The band reject filter of claim 8 wherein said waveguide has a
cross-sectional width of about 0.75 inch and cross-sectional length
of about 0.375 inch for said waveguide cutoff frequency of about
7.88 GHz.
10. The band reject filter of claim 9 wherein said waveguide is a
curved waveguide.
11. The band reject filter of claim 9 further comprising:
at least one tuning stub disposed through said second block and
said second wall and opposite said cavity for impedance matching to
said cavity.
12. The band reject filter of claim 4 further comprising:
a plurality of cavities formed in said first block, said plurality
of cavities including said at least one cavity, and each of said
plurality of cavities operating in the evanescent mode.
13. The band reject filter of claim 12 wherein at least two of said
plurality of cavities have the same type of cross-section.
14. The band reject filter of claim 13 wherein said at least two of
said plurality of cavities have circular cross-sections with the
same diameters.
15. The band reject filter of claim 13 wherein said at least two of
said plurality of cavities have circular cross-sections with
different diameters.
16. The band reject filter of claim 12 further comprising:
a plurality of tuning stubs disposed through said second block and
said second wall and opposite said plurality of cavities for
impedance matching said cavities.
17. The band reject filter of claim 2 further comprising:
a plurality of cavities formed in said first block, said plurality
of cavities including said at least one cavity, each of said
plurality of cavities operating in the evanescent mode; and
a plurality of tuning stubs disposed through said second block and
said second wall and opposite said plurality of cavities for
impedance matching said cavities.
18. The band reject filter of claim 17 wherein said at least two of
said plurality of cavities are different in either cross-section
type or dimension from each other.
19. The band reject filter of claim 17 wherein said waveguide is a
curved waveguide.
20. A method of making an evanescent mode band reject filter
comprising a waveguide coupled to a plurality of cutoff cavities,
said method comprising the steps of:
providing a first block having a first surface forming a first wall
of a waveguide, said first block including plurality of cutoff
cavities formed therein from said first surface, said plurality of
cutoff cavities directly coupled to said waveguide;
providing a second block having a second surface, a third surface
and a fourth surface, said second surface to form a second wall of
said waveguide, said second wall to be opposite to said first wall
in said waveguide, and said third and fourth surfaces forming
opposite side walls of said waveguide and to be perpendicular to
said first and second walls of said waveguide; and
connecting said first block and said second block together such
that said second surface and said first surface face each other to
form said waveguide, wherein each of said plurality of cutoff
cavities operates in an evanescent mode such that said plurality of
cutoff cavities have cavity cutoff frequencies above the stopband
frequency of said evanescent mode band reject filter.
21. The method of claim 20 wherein said plurality of cutoff
cavities are substantially cylindrical cavities with a circular,
elliptical, or substantially rectangular cross-section.
22. The method of claim 21 wherein said first block providing step
includes providing said first block comprising a solid metal block
and forming said plurality of cutoff cavities formed therein by
milling said cavities into said solid metal block.
23. The method of claim 22 further comprising the step of:
forming a plurality of holes through said second wall of said
waveguide for a plurality of stub tuners to be disposed
therethrough, such that each of said plurality of cutoff cavities
is to be substantially opposite a corresponding one said plurality
of stub tuners.
24. The method of claim 21 wherein said first block providing step
includes providing a molded metal block having cavities formed
therein.
25. The method of claim 24 further comprising the step of:
forming a plurality of holes through said second wall of said
waveguide for a plurality of stub tuners to be disposed
therethrough, such that each of said plurality of cutoff cavities
is to be substantially opposite a corresponding one said plurality
of stub tuners.
26. The method of claim 20 wherein said connecting step is
accomplished by providing a plurality of through-holes through
edges of said first block and of said second block, and using a
plurality of screws or bolts through said plurality of
through-holes to connect said first and second blocks together.
27. The method of claim 21 wherein at least two of said plurality
of cutoff cavities have the same type of cross-section as each
other.
28. The method of claim 21 wherein at least two of said plurality
of cutoff cavities have a circular cross-section.
29. The method of claim 28 wherein said at least two of said
plurality of cutoff cavities have the same diameter.
30. The method of claim 28 wherein said at least two of said
plurality of cutoff cavities have different diameters.
31. The method of claim 21 wherein at least one of said plurality
of cutoff cavities has slightly inwardly slanted walls.
Description
BACKGROUND OF THE INVENTION
The present invention relates to microwave transmission systems.
More specifically, the present invention relates to evanescent mode
band reject filters suitable for use in microwave (or
millimeter-wave) transmission systems. Embodiments of the present
invention are particularly useful for providing easily
manufactured, good performance band reject filters utilizing
evanescent mode cavities.
Band reject filters are commonly used in microwave transmission
systems to minimize or attenuate propagation of a certain band of
frequencies within a stopband bandwidth starting from a stopband
frequency. The conventional method of designing such a band reject
filter involves coupling a waveguide to a series of cavities, where
these cavities are coupled to the waveguide via coupling apertures.
In order to operate properly, these filters require cavities having
a depth that is approximately a half-wavelength of the stopband
frequency of the band reject filter. These cavities operate in a
propagating mode, i.e., the cutoff frequency of the cavities is
below the stopband frequencies of the filter. Conventional
propagation mode band reject filters are thus designed using a
waveguide coupled via apertures to cavities which operate in a
normal propagating mode.
With such conventional propagation mode band reject filters,
performance characteristics depend critically on specific
dimensions. Specifically, a multitude of critical dimensions limit
the performance in such filters. For example, the stopband
bandwidth is controlled by the aperture dimensions in these filters
having a waveguide with a wall having apertures coupled to
cavities. In addition, the stopband frequency is controlled by the
cavity depth, which must be about a half-wavelength long. Further,
low voltage standing wave ratio (VSWR) at the passband is
controlled by the spacing between cavities. These critical
dimensions in the filter, and in particular aperture dimensions
(such as slots having curved and/or straight edges in the wall of
the waveguide), are often difficult to manufacture in order to
provide reliable devices. The dependence of performance on these
many critical dimensions reduces the device manufacturability of
conventional propagation mode band reject filters.
From the above, it is seen that an easy-to-manufacture and high
performance band reject filter with minimized dependence on
multiple critical dimensions is desirable.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and methods for an
easily manufactured evanescent mode band reject filter that
provides high performance with minimal dependence on critical
dimensions.
According to one embodiment, the present invention provides a band
reject filter including a waveguide having an input, an output, a
first wall between the input and the output, and a second wall
opposite the first wall. The first wall is pan of a substantially
solid first block, and the second wall is part of a substantially
solid second block. The waveguide is capable of transmitting an
electromagnetic radiation signal, such as a microwave or
millimeter-wave signal in specific embodiments, from the input to
the output, where the signal is at an operating frequency above a
waveguide cutoff frequency. The band reject filter also includes at
least one cavity coupled directly to the first wall of the
waveguide, where the cavity is a substantially cylindrical cavity
formed in the first block. Further, the cavity operates in an
evanescent mode such that the cavity has a cavity cutoff frequency
above the stopband (or rejection band) frequency of the band reject
filter. The cavity may have a circular, elliptical, or
substantially rectangular cross-section in some specific
embodiments.
According to another embodiment, the present invention provides a
method of making an evanescent mode band reject filter that
includes a waveguide coupled to multiple cutoff cavities. The
method includes the step of providing a first block having a first
surface forming a first wall of a waveguide. The first block
includes multiple cutoff cavities formed therein from the first
surface, and the multiple cutoff cavities are directly coupled to
the waveguide. The method also includes the step of providing a
second block having a second surface, a third surface and a fourth
surface. The second surface forms a second wall of the waveguide,
where the second wall is to be opposite to the first wall of the
waveguide. The third and fourth surfaces form opposite side walls
of the waveguide, where the side walls are to be perpendicular to
the first and second walls of the waveguide. Further, the method
includes the step of connecting the first block and the second
block together such that the second surface and the first surface
face each other to form the waveguide. In some embodiments, the
method further includes a step of providing multiple holes through
the second wall of the waveguide, where multiple stub tuners are to
be disposed through the multiple holes and each of the cutoff
cavities is to be substantially opposite a corresponding one of the
stub tuners.
These and other embodiments of the present invention, as well as
its advantages and features are described in more detail in
conjunction with the text below and attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) is an exterior perspective view of an assembled
evanescent mode band reject filter, according to a specific
embodiment of the present invention;
FIG. 1(b) shows a top perspective view of the upper part and the
lower part of the unassembled evanescent mode band reject filter of
FIG. 1(a);
FIG. 2(a) is an exterior perspective view of an assembled curved
evanescent mode band reject filter, according to another specific
embodiment of the present invention;
FIG. 2(b) is a top perspective view of the upper part and the lower
part of the unassembled curved evanescent mode band reject filter
of FIG. 2(a);
FIG. 3 is a graph showing the measured S.sub.11 and S.sub.21
performance of the evanescent mode band reject filter of FIG. 2(a),
according to a specific embodiment; and
FIG. 4 is a graph showing on a magnified scale the measured
S.sub.21 performance of the evanescent mode band reject filter of
FIG. 2(a), according to a specific embodiment.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
The present invention provides an evanescent mode band reject
filter designed using a waveguide coupled directly to cavities
operating in the evanescent mode. In evanescent mode band reject
filters according to the present invention, cutoff cavities (i.e.,
cavities having cutoff frequencies above the stopband frequencies
of the band reject filters) are used, in contrast to normal
propagating mode cavities which are used in conventional band
reject filters. Unlike the conventional propagating mode band
reject filters' apertures or slots, which often are shaped such
that device manufacturability undesirably becomes an issue of
device performance, the present invention eliminates slots and has
cavities directly coupled to the waveguide without use of any
strangely-shaped apertures or slots in the wall of the waveguide
adjacent to the cavities. Further, with the present invention, the
location of the stopband is controlled only by the diameter of the
cavities, and the depth of the cavity is not critical. In some
embodiments, tuning elements such as tuning stubs can be utilized
for further improvement in filter performance. With the present
invention, the number of dimensions critical to performance is
reduced, improving manufacturability and allowing improved filter
response, as discussed further below.
FIG. 1(a) is an exterior perspective view of an assembled
evanescent mode band reject filter 10, according to a specific
embodiment of the present invention. As seen in FIG. 1(a),
assembled evanescent mode band reject filter 10 includes an upper
part 15 and a lower part 20, which may be secured to each other by
fasteners 25 such as screws (or bolts) going through holes (not
seen in FIG. 1(a)) disposed through upper part 15 and lower part
20. Both upper part 15 and lower part 20 are made of conducting
material such as copper, aluminum, or stainless steel (preferably
Invar). When assembled, upper part 15 and lower part 20 form a
waveguide having interior walls 30, 35, 40 and 45. Walls 30, 35 and
40 are formed from lower part 20, while wall 45 is formed from
upper part 15. The rectangular cross-sectional waveguide made of
walls 30, 35, 40 and 45 has a width of about 0.75 inch and a height
of about 0.375 inch, in a specific embodiment where the cutoff
frequency of the dominant TE.sub.10 mode in waveguide is about 7.88
Gigahertz (GHz). The waveguide of filter 10 has flanged ends 50
with holes 55 therethrough for fasteners such as screws or bolts
(not shown) so that filter 10 may be connected to other elements in
a microwave (or millimeter-wave) transmission system. In the
present specific embodiment, the waveguide is filled with air, but
the waveguide may be filled with different materials in other
embodiments.
FIG. 1(b) is a top perspective view of upper part 15 and lower part
20 of the unassembled evanescent mode band reject filter 10 of FIG.
1(a). As seen in FIG. 1(b), upper part 15 includes cutoff cavities
70, and lower part 20 includes tuning stubs 75 corresponding to
each cutoff cavity 70. As seen in FIG. 1(b), upper part 15 is a
substantially solid block having cutoff cavities 70 formed therein.
Upper part 15 has a height (h) and a minimal width (w) sufficient
to provide cutoff cavities 70 formed in the solid block. The solid
block of upper part 15 extends beyond w at the sides to provide
flanges having holes 80 for fasteners 25 to secure and facilitate
attachment to lower part 20, which also has holes 85
correspondingly.
As seen in FIG. 1(b), each cavity 70 is a circular substantially
cylindrical cavity having a circular cross-section and cavity walls
75 is formed in wall 45 of upper part 15. The circular
cross-section of each cavity 70 has a diameter of about 13.5
millimeters (mm), which is less than the width of the waveguide of
filter 10, according to the specific embodiment. Cavity walls 75
are substantially parallel to walls 30 and 40 in the specific
embodiment, and provide a cavity depth of about 18 mm. Of course,
the cavity depth should be less than h, which is about 20 mm in the
specific embodiment. In some embodiments, cavity walls 75 may be
slightly slanted inward towards the center of the corresponding
cavity 70 to facilitate manufacturing of filter 10. Cavity depth,
although not critical to filter performance, preferably should not
be less than the diameter of cavity 70. However, cavity depth may
be less than the diameter of cavity 70 in other embodiments.
According to the specific embodiment, filter 10 has a length of
about 140 mm to accommodate four cutoff cavities 70. In accordance
with other specific embodiments, longer filters with more cavities
will generally result in a wider stopband bandwidth and increased
rejection over the stopband, as compared to shorter filters with
fewer cavities. For other specific embodiments, each cavity 70 may
have different diameters to provide a band reject filter with a
wider stopband bandwidth, as compared to a filters where each
cavity has the same diameter.
Each cutoff cavity 70 is separated from an adjacent cutoff cavity
70 by a distance (d.sub.c measured between respective centers of
each cavity 70) of about 30 mm in the specific embodiment. As
mentioned above, the location of the stopband of filter 10
advantageously is controlled by the diameter of the cavities,
rather than being dependent on the oftentimes strangely-shaped
dimensions of apertures used in conventional propagation mode band
reject filters. Unlike conventional propagation mode band reject
filters where the depth of the cavities determines the stopband
frequency and the spacing between cavities controls passband VSWR,
the depth of cavities 70 and the spacing between cavities 70 are
not critical in evanescent mode band reject filter 10 of the
present invention. In accordance with other embodiments, the band
reject filter may have a different d.sub.c between different
adjacent cavities.
In some embodiments of filter 10, each cavity 70 may produce some
inductance which can be matched by the use of the corresponding
tuning stub 75. Each tuning stub 75 is separated from an adjacent
tuning stub 75 by about d.sub.c, since each tuning stub 75 is
located substantially at the center of its corresponding cavity 70.
Because each cutoff cavity 70 and corresponding stub 75 can be
matched independently of the other cavity/stub pairs, minor
variations in individual filters 10 due to manufacturing tolerances
do not result in filter-to-filter performance problems that are
often encountered with other conventional band reject filters. It
is recognized that other specific embodiments may not require the
use of stub tuners. For the filters discussed above and below
according to the present invention, upper and parts of the filters
may be easily formed by milling cavities and/or partial waveguides
with stub tuner through-holes into solid metal blocks, or by
providing molded metal blocks having cavities and/or partial
waveguides with stub tuner through-holes formed therein. The parts
of these filters may thus be manufactured fairly easily without
having to create complex apertures or manually putting together
complicated structures to make high performance filters.
Accordingly, manufacturing is facilitated with the present
invention.
It is recognized that although the specific embodiments described
above and below have specific dimensions appropriate for use in
microwave transmission systems, other embodiments with different
dimensions appropriate for use in millimeter-wave transmission
systems are also within the scope of the invention. Further, the
specific embodiments described above and below have substantially
cylindrical cavities having a circular cross-section, but other
embodiments of the invention may have cavities with elliptical or
substantially rectangular cross-sections. It is also possible that
the cavities in the same filter may have different cross-sections,
depths, dimensions and other variations from each other to provide
a filter having a combination of different types of cavities.
FIGS. 2(a) and 2(b) illustrate another specific embodiment, similar
to the specific embodiment of FIGS. 1(a) and 1(b) except having a
bend or curve instead of being straight. FIG. 2(a) is an exterior
perspective view of an assembled curved evanescent mode band reject
filter, according to another specific embodiment of the present
invention. As seen in FIG. 2(a), assembled curved evanescent mode
band reject filter 100 includes a curved upper part 105 and a
curved lower part 110, which may be secured to each other by
fasteners 115 such as screws or bolts going through holes (not seen
in FIG. 2(a)) disposed through upper part 105 and lower part 110.
When assembled, upper part 105 and lower part 110 form a curved
waveguide having interior walls 120, 125, 130 and 135. Walls 120,
125 and 130 are formed from lower part 110, while wall 135 is
formed from upper part 105. The rectangular cross-sectional
waveguide made of walls 120, 125, 130 and 135 has a width of about
0.75 inch and a height of about 0.375 inch, in the specific
embodiment where the cutoff frequency of the waveguide is about
7.88 GHz. Of course, for other embodiments, the waveguide
dimensions will vary for different cutoff frequencies. The
waveguide of filter 100 has flanged ends 140 with holes 145
therethrough for fasteners like screws or bolts (not shown) so that
curved filter 100 may be connected to other elements in a microwave
transmission system.
Generally, curved filter 100 is useful for connecting elements in
transmission systems which have space constraints. Although curved
filter 100 shown in FIGS. 2(a) and 2(b) has a specific curvature
and dimensions, various other curvature types and dimensions also
may be used in other embodiments.
FIG. 2(b) is a top perspective view of upper part 105 and lower
part 110 of the unassembled curved evanescent mode band reject
filter 100 of FIG. 2(a). As seen in FIG. 2(b), upper part 105
includes cutoff cavities 150, and lower part 110 includes tuning
stubs 155 corresponding to each cutoff cavity 150. As seen in FIG.
2(b), upper part 105 is a substantially solid curved block having
cutoff cavities 150 formed therein. Upper part 105 has a height (h)
and a minimal width (w) sufficient to provide cutoff cavities 150
formed in the curved solid block. The curved solid block of upper
part 105 extends beyond w at the sides to provide flanges having
holes 160 for fasteners 115 to secure and facilitate attachment to
lower part 110, which also has holes 165 correspondingly.
Filter 100 shown in FIGS. 2(a) and 2(b) have cavity dimensions
similar to those discussed above for filter 10 shown in FIGS. 1(a)
and 1(b), with similar advantages. In general, curved evanescent
mode band reject filter 100 and straight filter 10 exhibit
comparable performance. As an example of typical filter
performance, FIGS. 3 and 4 are graphs illustrating performance
measurements of evanescent mode band reject filter 100 from 10 GHz
to 15 GHz, according to the specific embodiment in FIG. 2(a).
In particular, FIG. 3 is a graph showing the measured S.sub.11
performance and the measured S.sub.21 performance over the measured
frequency range. The return loss, S.sub.11, which is proportional
to the input VSWR, is the ratio of power reflected at the filter
input to the power input to the filter input. S.sub.11 is indicated
by line 200 and is shown on a 5 decibel (dB)/unit scale with the
reference at -20 dB. The transmission loss, S.sub.21, is the ratio
of power output at the filter output to the power input to the
filter input. S.sub.21 is indicated by line 250 and is shown on a
10 dB/unit scale with the reference being at -40 dB. For a high
performance band reject filter, it is desirable that S.sub.11 be
low (i.e., low power reflection at the input) and S.sub.21 be high
(i.e., good transmission or low insertion loss) for passband
frequencies, and that S.sub.21 be low (i.e., good band rejection)
at stopband frequencies.
As seen in FIG. 3, specific measurements of S.sub.11 and S.sub.21
at specific frequencies were taken, as shown in Tables 1 and 2,
respectively, that indicate that filter 100 has good input VSWR and
transmission performance at the passband (about 10.95 GHz to about
12.75 GHz) and excellent band rejection performance over the
stopband (between about 14.0 GHz to about 14.5 GHz).
TABLE 1 ______________________________________ Return Loss
Characteristics (S.sub.11) Frequency (GHz) S.sub.11 (dB)
______________________________________ 10.95 -23.36 12.20 -28.55
12.75 -25.47 ______________________________________
TABLE 2 ______________________________________ Transmission Loss
Characteristics (S.sub.21) Frequency (GHz) S.sub.21 (dB)
______________________________________ 10.95 -0.07 12.20 -0.06
12.75 -0.15 14.0 -43.67 14.5 -49.44
______________________________________
FIG. 4 is a graph showing the measured S.sub.21 performance of
evanescent mode band reject filter 100 of FIG. 2(a), according to
the specific embodiment. Specifically, FIG. 4 shows the measured
S.sub.21 on a magnified scale over the measured frequency range of
10 GHz to 15 GHz in order to show the ripple across the passband of
filter 100. In FIG. 4, S.sub.21 is indicated by line 300 and is
shown on 0.2 dB/unit scale with the reference being at 0 dB. For a
high performance band reject filter, it is desirable that S.sub.21
be high and have minimal ripple over the passband frequencies, in
addition to S.sub.21 being low at stopband frequencies. As seen in
FIG. 4, specific measurements of S.sub.21 at specific frequencies
were taken that indicate that filter 100 has good transmission
performance with minimal ripple (less than 0.2 dB) at the passband
(about 10.95 GHz to about 12.75 GHz).
It is to be understood that the above description is intended to be
illustrative and not restrictive. Many embodiments remaining within
the scope of the claims of the present invention will be apparent
to those of skill in the art upon reviewing the above description.
For example, although the specific embodiment shows dimensions for
a particular stopband frequencies, other embodiments will have
different dimensions for other stopband frequencies. Although the
specific embodiments illustrate filters at about microwave
frequencies, other embodiments may be filters at millimeter-wave
frequencies. In addition, although the specific embodiments have
upper and bottom parts of the filters connected using fasteners
like screws or bolts, other types of fastening mechanisms such as
clamps, clips, epoxy, etc. also may be used in other embodiments.
Further, the specific embodiments show filters using four cutoff
cavities, however other embodiments may utilize fewer or more
cutoff cavities for different applications. Still further, the
specific embodiments illustrate cutoff cavities having a particular
diameter, but other diameters may be used in other embodiments with
different requirements. Still further yet, other embodiments may
have a combination of cutoff cavities with varying cross-sections,
depth, diameter, separation, etc. The scope of the inventions
should, therefore, be determined not with reference to the above
description, but should instead be determined with reference to the
appended claims, along with the full scope of equivalents to which
such claims are entitled.
* * * * *